Field of the Invention
The present invention relates to a radiation imaging apparatus, a determination apparatus, a method of controlling the radiation imaging apparatus, a calibration method, and a storage medium.
Description of the Related Art
An X-ray imaging system as an example of a radiation imaging system includes an X-ray generator which generates X-rays and an X-ray imaging apparatus which generates an X-ray image upon receiving X-rays. Recently, X-ray imaging apparatuses using FPDs (Flat Panel Detectors) in place of conventional X-ray films or imaging plates have been popular on the market. An FPD has detection elements (pixels) arranged in a matrix pattern on the detector surface, and is configured to accumulate signal charges generated for the respective pixels in accordance with the amount of X-rays detected and generate a signal value of digital data upon A/D conversion after the accumulation. An image obtained by performing image processing to this digital data is used for diagnosis and the like.
An X-ray imaging apparatus using such an FPD is generally used while being fixed to a stand or table. It is, however, sometimes necessary to perform imaging while using an X-ray imaging apparatus in free unfixed positions, depending on the imaging technique to be used. In order to meet such requirements, an apparatus made easy to handle by reducing the thickness and weight of a housing accommodating an FPD has also been commercialized. Such an apparatus can be used while being fixed on a stand or table as in a conventional technique. In addition, the apparatus is used while being placed on a bed or used while being held by a patient as is often practiced in orthopedic surgery. This makes it possible to perform imaging operation which cannot be done in a fixed position. Furthermore, the apparatus can be applied to various usages exploiting portability, for example, bringing out the apparatus from a hospital for use at the time of an accident or disaster and imaging a home health care patient at his/her house.
A conventional X-ray imaging apparatus, however, needs to match the timing of X-ray irradiation with that at which the FPD accumulates signal charges originating from X-rays. This requires an interface or signal cable for electrically connecting synchronization signals between the X-ray generator and the X-ray imaging apparatus.
In order to solve this problem, Japanese Patent Laid-Open No. 2002-543684 has proposed a technique of eliminating the necessity of connection of synchronization signals to an X-ray generator by allowing an X-ray imaging apparatus by itself to detect X-ray irradiation and shift an FPD from a normal mode to a mode of accumulating signal charges originating from X-rays or a subsequent readout mode. This is a scheme of detecting X-ray irradiation by monitoring a bias current in the FPD which varies due to X-ray incidence.
On the other hand, electrical components cannot generally avoid the influence of disturbances, and the X-ray imaging apparatus also generates noise due to a received impact/vibration/acceleration. As a consequence, internal electrical signals are influenced by noise more than a little. A bias current in the FPD is susceptible to the influence of noise because it handles weak signals. Malfunction due to noise leads to false X-ray detection. In practice, therefore, the apparatus executes imaging in spite of no detection of X-rays. In this case, it is necessary to discard images (misshooting) or perform re-imaging operation. This disturbs the work flow of a radiation technician.
In contrast to this, Japanese Patent Laid-Open No. 2012-110565 discloses a technique of setting an FPD in an accumulation state upon determining the start of X-ray irradiation when the electrical signal output from the FPD exceeds a predetermined threshold in a readout state, determining, from the differential value of subsequent FPD outputs, whether the determination of the start of X-ray irradiation is correct, interrupting the accumulation state upon determining that the determination is incorrect, and resuming the determination of the start of X-ray irradiation based on FPD outputs in the readout state.
The technique disclosed in Japanese Patent Laid-Open No. 2012-110565 is configured to perform X-ray irradiation determination by comparing the electrical signal obtained by converting the charges generated in pixels with a threshold. This technique then re-determines, based on a temporal change in the electrical signal after X-ray irradiation determination, whether the electrical signal truly originates from X-ray irradiation or noise, and interrupts accumulation operation which has been started, if determining that the signal originates from noise.
Japanese Patent Laid-Open No. 2012-100807 discloses a similar technique, which is configured to disconnect the operation switching unit of an FPD from an X-ray irradiation detection unit if the detection result obtained by an acceleration sensor which detects acceleration becomes equal to or more than a detection threshold, and invalidate X-ray irradiation detection for a predetermined period.
However, the technique disclosed in Japanese Patent Laid-Open No. 2012-110565 detects false detection by posteriorly re-evaluating the result obtained once by X-ray irradiation determination, interrupts accumulation operation which has been started once, and returns to an X-ray irradiation wait state again. For this reason, it is impossible to perform X-ray irradiation determination at least during return operation. This influences the work flow of a radiation technician more than a little. In addition, upon receiving disturbances almost simultaneously with X-ray irradiation, this technique may determine by re-evaluation that the X-ray irradiation determination result indicates false determination, in spite of actual X-ray irradiation. In this case, the patient is unnecessarily exposed to X-rays.
In addition, according to Japanese Patent Laid-Open No. 2012-100807, it is impossible to perform X-ray irradiation detection for a predetermined period after the detection of acceleration. For this reason, if the apparatus performs X-ray irradiation during this period, an imaging error occurs because the apparatus does not shift to accumulation operation. As a result, the patient may be unnecessarily exposed to X-rays.
In consideration of the above problems, the present invention provides an apparatus which reduces false radiation irradiation detection due to disturbances such as acceleration/impact without disturbing the imaging work flow of a radiation technician, and reduces invalid radiation exposure on a patient in a radiation detector having a radiation detection mechanism which shifts the operation mode of the radiation detector to the accumulation mode by itself upon detecting a radiation irradiation state.